Process heater



S. A. GUERRIERI PROCESS HEATER Filed Sept. 2, 1964 HEAT FLUX TO FRONT OFTUBE D Q) Q 5 SheecsShee1; 2

v I l 'w/o =20 I ,3 I I [I w/o 1.5 I

W/D=0 I", z I

FIG. 5

INVENTOR. SALVATORE A. GUERR/ER/ A T TORNE V5 1966 s. A. GUERRIERI3,267,910

PROCESS HEATER Filed Sept. 2, 1964 5 S eets-S eet 5 INVENTOR. SALVATOREA. GUERR/ER/ A T TORNE VS United States Patent 3,267,916 PROQESS HEATERSalvatore A. Guerrieri, Rowayton, Conn, assignor to The Lurnmns Company,New York, N.Y., a corporation of Delaware Filed Sept. 2, 1964, Ser. No.393,989 4 Claims. (Cl. 122-356) This invention relates, in general, to anew and improved process furnace utilizing finned radiant tubes and,more particularly, it relates to a process furnace in which finnedradiant tubes are employed to control the distribution of the heat fluxabout the periphery of process tubes.

Process furnaces comprise tubes heated in a chamber either by thecombustion of fuel within the chamber, by a radiator, by hot combustionproducts entering the chamber, or by a combination of these means. Thesefurnaces differ from boilers (i.e., steam generators) because of thedifierence in the physical properties of the fluids flowing in the tubesand because of the nature of the process, at least for boilers operatingbelow the critical temperature of water.

In the case of boilers, the fluid inside the tubes is inert and has ahigh thermal conductivity. Therefore, heat transfer coefficients insidethe tubes are high, and high heat fluxes are possible with moderatetube-to-water temperature dilferences. Because of this, local hot spotsare no problem. In the case of process heaters, on the other hand, thefluids inside the tubes are temperature sensitive and have relativelylow thermal conductivities. Therefore, the heat transfer coefficientsare also relatively low.

Further, in many modern process furnaces, chemical reactions areintended to take place inside the tubes, and these reactions aretemperature sensitive as to yield and quality of product. It is usuallynecessary to design into the furnace a definite heat flux pattern andtemperature patttern for the process fluid in the tubes. These processrequirements call for very careful design of process furnaces and thearrangement of the tubes. Many designs of furnaces and burners have beendeveloped to achieve these ends. In all of these designs, the attempt ismade to control the temperature and heat flux along the tube as well asaround the tube. However, the independent control of temperature andheat flux along the tube and around the tube has not yet been obtainedeconomically. Circumferential heat flux may be improved somewhat if thetubes in the radiant section of the furnace are arranged in a singlerow, and the plane of the tubes is irradiated from planes on both sidesof the tube plane. However, even when the two radiating surfacesapproach the ideal of being uniform in temperature and heat flux overthe entire area, circumferential heat flux around the tubes is far fromuniform, heat flux at the sides of the tubes often being as little as0.5 to 0.6 of the heat flux at the points directly opposite to theradiating planes. Thus, ideal radiating planes may provide asubstantially uniform heat flux and temperature along the length of agiven process tube, but will have little effect on the uniformity ofheat flux around the periphery of the tube. On a practical level, ofcourse, it is much more expensive to provide a heater with two radiatingsurfaces, and with only a single row of process tubes, throughput iskept low unless the furnace is very large.

Thus, it would be advantageous to be able to achieve a uniform orcontrolled heat flux about the periphery of process tubes in a processfurnace when radiated from a source located at one side only.

Accordingly, it is the general object of this invention to provide a newand improved process heater which is more economical to manufacture andwhich can be designed for substantially uniform peripheral heat flux tothe tubes.

Another object of this invention is the provision of a new and improvedprocess heater capable of utilizing direct and indirect radiation from asingle source to equalize the heat flux about the tubes.

Still another object of the invention is the provision of a new andimproved process heater capable of greater throughput per unit of heatenergy, and in which heat flux both around the tubes and along theirlength is subject to positive control.

Various other objects and advantages of the invention will become clearfrom the following description of several embodiments of the invention,and the novel features will be particularly pointed out in connectionwith the appended claims. A better understanding of the invention willbe gained by referring to the following description in conjunction withthe accompanying drawings, which are illustrative only and are not to beinterpreted in a limiting sense, and in which:

FIGURE 1, is a cross sectional view of a process tube built inaccordance with the principles of the present invention;

FIGURE 2 is a cross sectional view of a second type of process tubebuilt in accordance with the principles of the present invention;

FIGURE 3 is a schematic representation of a conventional tube inaprocess furnace, with the radiant source at the bottom of the drawing;

FIGURE 4 is a schematic representative of process tubes of the presentinvention in the same environment as the process tubes of FIGURES 3;

FIGURE 5 is a graph showing the effect of the tube spacing ratio of thedirect heat flux to the front face of the tube relative to the heat fluxto the back face of the tube, for plain tubes and tubes with variousfins;

FIGURE 6 is a cross sectional elevation of a pyrolysis heater built inaccordance with the principles of the present invention;

FIGURE 7 is a partial cross sectional plan view of the heater of FIGURE6 taken along lines 7-7;

FIGURE 8 is a cross sectional elevation of another furnace built inaccordance with the principles of the present invention; and

FIGURE 9 is a cross sectional elevation of a process heater with acentral downdraft convection section built in accordance with theprinciples of the present invention.

In FIGURE 1, there is shown a process tube 10 built in accordance withthe principles of the present invention. The process tube 10 has atubular main body portion 12, the front face 14 thereof being adapted toface a source of radiant heat energy. A fin 16 is welded to the b ackface 18 of the main body 12. Fin In is shown as a flat piece, but undercertain circumstances a curved fin may be preferred. The fin 16 extendsthe length of the tube 14 and has a width which varies in accordancewith the diameter of the tube body 12. The fin 16 is preferably made ofa material having high thermal conductivity and an emissivityapproaching unity. This is because heat flux to the fin 16 llS byradiation whereas heat flow from the fin 16 to the tube is mostly byconduction, although in some instances a significant fraction of the fint o tube heat flow may be by reradiation. In the alternative, in orderto obtain maximum fin efilciency, the fin can be coated with a materialof high emissivity so that substantially all of the radiant heatstriking the fin will be absorbed with little reflected.

In FIGURE 2, there is shown a sec-0nd embodiment 10' of the presentinvention in which a-tubular main body 12' is utilized having a frontface 14'. Two fins 24 and 26 are welded to the back face 18 of the tube12' along suitable welded joints 27 and 28. The fins 24 and 26 havetheir overall width (from tip to tip) varied in accordance with thediameter of the tube 12' in the same manner as the width of fin 16 isvaried with respect to the 3 tube 12. The fins 24 and 26 extend thelength of the tube 12', and are cut transversely at intervals to avoidhigh temperature stresses and warpage.

In both FIGURES 1 and 2, the radiating source is illustrated as beingbelow the tubes. Both the tubes and the fins absorb radiation incidentupon them and transmit the heat received to the fluid flowing within themain bodies 12 and 12'. In the case of the main bodies 12 and 12, thetransfer is directly through the walls of the tubes, whereas in the caseof the fins 16, 24 and 26 the heat received first flows along the finsand then into the tubes through the welded areas.

In FIGURE 3, the mechanism of radiant heat transfer to the face of atube away from the flame and to the face of the tube adjacent the flameis shown for a standard plain tube. In FIGURE 3, two tubes 30 and 30'are shown having a face 32 and 32' closest to a source 34 of radiantheat. Behind the tubes 30 and 30' there is a refractory wall 36 whichreflects or reradiates heat from the source 34. It can be easily seenfrom the arrows denoting the direction of the radiated heat from thesource 34 that some of the heat flux passes between the tubes 30 and 30and is reflected back between the tubes 30 and 30 as well as to the backfaces 38 and 38' of the tubes.

Some of the thermal rays from the source 3-4 are interrupted andabsorbed by the tubes 30 and 30'. If there is no heat loss through therefractory wall 3 6, all of the heat from the source 34 is eitherreradiated or reflected. As in the case of the direct radiation, thisreradiation and refleotion is partly intercepted by the tubes 30 and 30and a portion of this reradia-tion and reflected energy passes throughthe spaces between the tubes. It is obvious that the heat received bythe portions 38 and 38 of the tubes facing the reradiating wall isprimarily by reradiation. The relative intensity of the heat received bythe front and rear faces 32, 32 and 38, 38' of the tubes 30 and 30'respectively depends primarily on the spacing of the tubes. The closerthe tube spacing, the greater the ratio of the heat flux on the frontrelative to the rear. These characteristics are more clearly brought outin FIGURE 5.

In FIGURE 5, Curve 1 shows the ratio of the direct heat flux to thefront face 32 of the tube 30 relative to the indirect, reradiated andreflected heat flux to the back face 38 of the tube 30. It is to beobserved that for the normal tube spacing of two tube diameters, theheat flux to the back face is only about one third of that received bythe front face. Curve 1 and the other curves of FIGURE neglect theeffect of the tube wall as a redistributor of heat which tends to reducethe disparity in heat flow of the front face relative to the back faceof the tube.

In FIGURE 4, there are shown two finned tubes 40 and 40' built inaccordance with the principles of the present invention and generallysimilar to the tube shown in FIGURE 1. The tubes 40 and 40 have a frontface 42 and 42 and a back face 44 and 44' respectively. Pins 46 and 46'are welded to the back faces 44 and 44 respectively of tubes 40 and 40.A source of thermal energy 47 directs thermal rays at the tubes 40 and40. Only three rays from a single point are shown, it being understoodthat rays will go in all directions from all points on the plane. Arefractory wall 49 is provided behind the tubes 40 and 40' whichreradiates or reflects the heat from the source 47. It will be notedthat the tubes 40 and 40' are on a common center line and are spaced agiven number of tube diameters from center to center thereof. The fins46 and 46 have a width W which is greater than the diameter D of thetubes 40 and 40. Thus, the ratio W/D is greater than unity.

Curves 2, 3 and 4 of FIGURE 5 show the ratio of direct and indirect heatflux to the fins 44, 44 relative to the direct heat fiux ltO the frontfaces 42., 42' of the tubes 40, 40 for different ratios of total finwidth to tube diameter. As it is to be expected, for a given tubespacing the ratio of the heat received by the fins to that received bythe tube increases with the fin width. It is of interest to observe thatfor a given value of W/D, the relative heat flux to the fins increaseswith tube spacing and above certain values the fins receive more heatthan the front face of the tubes. In other words, it is possible by theuse of suitable fin widths and tube spacing to reverse the normalconditions of heat flux and to find the back face of the tubes to bereceiving more heat (via fins) than the front face. However, it isusually the aim to equalize the heat flux front and back.

Curve 5 of FIGURE 5 is a cross plot of Curves 2, 3 and 4 for the casewhen the ratio of the front-to-back heat interchange factor is unity andshows the variation of tube spacing with W/D. Curve -6 is a similarcross plot for the case when the ratio is .8. As an example, if W/D=2.0,the tube spacing as read from Curve 5 should be 2.9 diameters, in orderto obtain equal heat flux to the front and back faces of the tube.

The utility of the invention may be illustrated by considering aspecific example of the advantages thereof when applied to a furnacehaving a single row of tubes spaced on two diameter centers, irradiatedfrom one side and backed up by a refractory wall on the other, whereinthe average heat flux is 12,000 Btu. (hr.) (sq. ft.). From Curve 1, theheat flux to the back of the tubes for a plain tube without the fins ofthe present invention is one third of that to the front half. Hence therate to the back half is /z of 12,000 or 6,000 B.t.u. (hr.) (sq. ft.),whereas the rate to the rate to the front face is 18,000 B.=t.u./ (hr.)(sq. ft.). If this row of tubes were replaced by a row of finned tubesat 2.9 diameters spacing and W/D=2.0 (to give equal flux front and back)and if the maximum rate of the plain tubes was acceptable, then eachsquare foot of tube of the finned tubes would be capable of receiving18,000 Btu/(hr.) (sq. ft.). But since these tubes occupy 2.9 times asmuch wall space as the plain tubes the flux for equivalent wall space is1 8,000 (2) /-(2.9) equals 12,400 B.t.u./ (hr.) (sq. ft.). Thus therewill be only a slight saving in refractory material, but a considerablesaving in tubes, since only two thirds as many finned tubes and returnbends will be required as plain tubes. If allowance is made for theeffect of the tube walls as redistributors of heat, somewhat closerspacing of the fintned tubes may be used than was used in the exampleand therefore, the finned tubes will show up to even better advantage.In the example of finned tubes shown, the direct heat flux on the tubeand iliu is about eighty four percent, whereas the indirect heat flux isabout thirteen percent of the total. It is obvious therefore, that thefinned tubes 40, 40 can be arranged in two parallel rows along thecenter lane of a combustion chamber fired from both sides and the heatof each of the rows could be substantially independently controlled.

In FIGURES 6 and 7 there is shown a process heater built in accordancwith the principles of the present invention. The heater 50 has acylindrical outer wall 52 and a cylindrical inner wall 54 defining anannular chamber '56 therebetween, with a refractory bottom ring 58 and arefractory top ring 60 enclosing the chamber 56. The inner and outerwalls '54 and 52 have refractory linings on the portion thereof exposedwithin the chamber 56. Combustion gases from chamber 56 pass throughduct 80 to convection unit 82 and thence to stack 84. The process heater50 is supported on suitable structural steel members '66.

Within th chamber 56 there are a plurality of groups of process tubeseach group comprising two rows of tubes 68 and 70. As seen in FIGURE 7,each group of tubes '68, 70 is on a radius of the annular chamber 56.Such an arrangement has certain advantages with respect to even heatingby burners 7-8 mounted in both the inner and outer walls. With thefinned process tubes of the invention this advantage is even greater,due to the fact that the two rows of tubes heated by any given rows ofburners form a uniquely controllable chamber, in

that radiation between adjoining rows of tubes, which are back-tobackwith respect to fin placement, is negligible.

As shown in FIGURE 6, process tubes 68, 70 are connected to headers 62,64 for passing process fiuids therethrough.

As noted hereinabove, it is desirable to have the fins 16 cuttransversely at regular intervals along their length to prevent adversethermal effects such as warping and the like, and this feature is alsoillustrated in FIGURE 6. The cutting may be done either before or afterthe fins 16 are installed on the process tubes 68, 70, depending on easeof fabrication and cost. The fins 16 will generally extend over theentire length of process tubes 68, 70 within chamber 5 6, though incertain applications only partial use of the fins may be desired.

Opposing burners 78 in any given segment of heater 50 are staggered sothat burners do not directly oppose one another. Valves 79 on eachburner and valves (not shown) for controlling the flow of fuel to groupsof burners provide a complete measure of control of the heat flux overthe length of the tubes. In operation, opposing burners 78 radiate heatinto a segment of chamber 56 and directly onto th front of process tubegroup 68 of one radial group and group 70' of another radial group. Thefins 1-6' keep this radiated heat substantially contained in a givensegment of chamber 56. Pins 16 insure that the circumferential heat fluxis substantially even around the periphery of the tubes, with the resultthat process fluids react (-or are heated) uniformly and completelyduring passage through the tubes, production is increased and recycleloads are decreased.

A more complete description of the heater illustrated in \FIGURES 6 and7 and described hereinabove will be found in my copending U.S. Patentapplication Serial No. 384,706, filed July 23, 1964, and entitled, APPA-RATUS.

In FIGURE 8, there is shown a cabin type furnace in vertical crosssection which utilizes the principles of the present invention. Here,too, the need for two, spaced radiating surfaces having a plurality ofburners therein, so as to more nearly achieve a uniform radiating planeon opposite sides of process tubes, has been eliminated by the use ofthe finned tubes of the present invention. That is, the cabin furnace'86 of FIGURE t3 has two groups of burners 88 and 90 positioned inparallel paths along the side edges of the bottom wall 92 of the furnace86. The groups of burners 88 and 90 are positioned along lines adjacentthe side walls 94 and 95 of the furnace. A suitable opening 98 isprovided adjacent the top of the furnace, which opening leads to theconvection section of the furnace. Of course, it is also possible toposition the burners along the side walls 94, 96, for firing directly atthe process tubes. In the furnace '86 there are positioned two groups100 and 102 of horizontally positioned process tubes. The group 100includes process tubes 104 each having a fin 106. The group of processtubes 102 includes process tubes 105 having fins 107 on the side thereoffurthest from side wall 96. The group of tubes 100 has the fins 106thereof on the side of tube 104 furthest from side wall 94. Tubes 104are closest to side wall 94 and tubes 105 are closest to side wall 96.The tubes 105 are staggered with respect to the tubes 104. In someinstances, fins 107 may overlap th space between the fins 106.Similarly, the fins 106 may overlap the space between adjacent fins 107in group 102. It can thus be seen that heat radiating from the burners88 which passes between the fins 106 of group 100 will be reflected and/or reradiated by the back sur faces of fins 107 of group 102. Similarly,heat radiating from burners 90 which passes between adjacent fins 107 ofgroup 102 will be reflected and/or reradiated by the fins 106 of thegroup 100. Although th tubes 104 and 105 have been shown as horizontallydisposed, it can easily be understood that in some instances, the tubesmay be better arranged vertically, as in FIGURES 6 and 7.

It will also be easily understood that in accordance with the teachingsof the present invention, the W/D ratio will determine the ratio of heatfiux supplied to the finned surface of tubes 104 and with respect to thediametrically opposite part of tubes 104 and 105.

FIGURE 9 is a cross sectional elevation of another furnace incorporatingthe principles of the present invention, which furnace includes acentral downdraft convection section.

In FIGURE 9, the furnace 108 is shown having two spaced parallelheatingchambers 1110 and 112 therein. The chamber is defined by a bottomwall 114, an outside side wall 116, and one vertical side wall 118 of acentral downdraft convection section 120. The top wall 122 defining thechamber 110 is inclined upwardly toward the center of the furnace 108 sothat combustion gases from burners 124, 126, i128 and mounted in thebottom wall 114 will be led to the upper opening 132 of the convectionsection .120. Burners 1-24, 126, 12 8 and 130 are arranged in parallelpaths parallel to side walls 116 and '1-18 and are spaced in parallelvertical planes intermediate three horizontally disposed process tubes1'34, 166 and 13 8 spaced above the bottom wall 114 and having fins 140,142 and 14-4 on the side thereof closest to top wall 122. The spacedhorizontal tubes '134, 136, and 138 are thus not immediately oppositeany of the burners 124, 12 6, 1 28 and 1-30, but receive the beat fiuxtherefrom as uniformly as possible. The fins 140, 142 and 144 have awidth greater than the diameter of th tubes 1-34, 136 and 1 3 8 and aredesigned so that the heat flux on the side of tubes 1'34, 136 and 1 3 8associated with fins 140, 142 and 144 is equal to the flux on thediametrically opposite side of the tubes facing the burners.

Chamber 112 is .a mirror image of the chamber 110 and has a bottom wall144, side walls 116' and 118', and a top wall 122'. Burners 124, 126,128' and 130' are mounted in the bottom wall 144' to direct the heat atsuitable horizontally disposed tubes 134, 136' and 138' having fins140', 142 and 144 on the side thereof closest to top wall 122. Top wall122' is inclined so that combustion gases from the burners 124', 126,128, and 130 will be directed to the downdraft convection section 120.Although a central downdraft convection section has been shown, it willeasily be understood than an updraft 'or sidedraft convection sectionmay also be utilized.

It will be understood that various changes in the details, steps,materials and arrangements of parts de scribed hereinabolve for purposesof illustrating the invention can be made by those skilled in the artwithout departing from the scop of the invention as set forth in theappended claims.

What is claimed is:

I. A process heater comprising,

a housing defining a chamber therein, two walls of said chamber beingparallel, refractory heat radiating and reflecting walls;

heating means along said two walls for heating said chamber;

a first group of process tubes spaced from each other and arranged in apath parallel to one of said two Walls;

a heat conductive fin on each of said tubes extending the length thereofon the side of said tubes furthest from said wall;

a second group of process tubes spaced from each other and arranged in apath parallel to the second of said two walls, said second group oftubes also having fins, the fins on said second group being on the sideof said tubes furthest from said second wall, said first and secondgroups of tubes thereby having fins on facing sides;

the width of said fins being greater than the diameter of said tubes andbeing controlled with respect to said diameter and to the spacing ofsaid tubes so as to equalize the heat flux from said heating means tothe point on said tubes nearest said heating means and the heat flux tothe fin side of said tubes.

2. The process heater as claimed in claim 1, wherein said first andsecond groups of tubes are spaced so as to be in staggered relation.

3. A radiant process heater comprising,

a housing defining an annular chamber having coaxial inner and outerWalls;

a plurality of burners in said inner and outer walls in spaced segmentsthereof;

a plurality of groups of process tubes with-in said chamber in spacedsegments between the segments of said burners, said tubes thus beingadapted to receive radiant heat from said burners;

a heat conductive tfin on each of said tubes extending the lengththereof;

the tubes in each said group comprising two rows, with said tins beingon the common side of said tubes between said two rows; and

means for passing process fluids through said tubes.

4. A radiant process heater comprising a housing defining a chamber;

a plurality of burners in the walls of said around the peripherythereof;

chamber References Cited by the Examiner UNITED STATES PATENTS 7/ 1948De Lorenzo 122356 3/1954 :Huet 122-667 6/1956 lPermann 1223 35 X 3/1960Wallis et al. 122333 6/1962 Dwyer 122-356 3/1965 Koniewiez 122-240 XFOREIGN PATENTS 4/1952 France.

CHARLES I. MYHRE, Primary Examiner.

4. A RADIANT PROCESS HEATER COMPRISING A HOUSING DEFINING A CHAMBR; APLURALITY OF BURNERS IN THE WALLS OF SAID CHAMBER AROUND THE PERIPHERYTHEREOF; A PLURALITY OF PROCESS TUBES ARRANGED IN TWO ROWS WITHIN SAIDCHAMBER, SAID TUBES BEING ADAPTED TO RECEIVE RADIANT HEAT FROM SAIDBURNERS; A HEAT CONDUCTIVE FIN ON EACH OF SAID TUBES EXTENDING THELENGTH THEREOF ON THE SIDE OF THE TUBE AWAY FROM THE SOURCE OF RADIANTHEAT; AND MEANS FOR PASSING PROCESS FLUID THROUGH SAID TUBES.